Heat pump seems to short cycle when reversing out of defrost

[jrudisill]

Hi,

I have a Nordyne split system heat pump, it's a T3QA, with a E1EH-012HA furnace, and a Honeywell CT3611 prog. t-stat.

The inside relay board (attached to the blower) arc'd over and blew out (corroded molex pins). The board was originally a 624-5680. I replaced it with the updated board, 624625.

Here's my question:

I'm thinking that before I replaced the relay board, when the heat pump went into a defrost cycle, when the cycle ended, the contactor didn't open, the compressor kept running, the reversing valve switched and the outside fan powered up all at once, with no delay. (I may be remembering wrong)

Anyhow, now, when it enters a defrost cycle, the strips power on, the valve reverses, outside fan powers down. Everything like normal.

When it reaches the end of the defrost cycle now, the strips turn off, the contactor opens and the compressor powers down for 25 seconds, and the reversing valve reverses. The inside blower keeps running. Then, after the 25 second delay, the contactor closes, and the compressor and outside fan powers up, back into a normal heat cycle.

Is this normal, the 25 second power-down on the compressor when reversing out of the defrost cycle? I'm worried something might have got spiked in the defrost board when the relay board got smoked. It seems to me like this would be short cycling the compressor.

Thanks, and hope I'm just being paranoid!

Jon

[Robert]

Hi Jon,

If the compressor is stopping and starting again after 25 seconds, this is not normal.

It also will likely burn compressor up eventually.


I would have an HVAC Tech come out immediately to diagnose problem and correct.



Thanks for inquiring,
Robert

[jrudisill]

Thanks for the reply.

Had a licensed heat pump tech come check it out. He ran it through a few defrost cycles, and said that it was a 30 second delay commanded by the defrost board, to let the pressures equalize when reversing out of defrost, so the compressor isn't hit with a hard load when the valve reverses.

He said he'd be glad to change out the defrost board if that's what I wanted, but he recommended against it, said that the way it was doing would help the compressor to last longer.

He also said that the major brand heat pumps don't do this, but that he'd seen some mobile home brands exhibit this behavior before.

Does this sound right, or should I get a second opinion?

[Robert]

Hi,

I haven't ran across one doing that as of yet, but could be as that does exist.

Either way, yes it is good, but 30 secs is not enough time and if that is what it's doing and reason, then it is not helping much.


I would take that out of circuit and wire in a timed delay relay to keep it off for 3-5 minutes instead of 30 secs.



Thanks,
Robert

[jrudisill]

I agree with you, It shouldn't be dropping out for 30 secs.

I may have found the culprit:

The defrost board is a 621579 (917178A). I looked at the instruction sheet for the board on this website:

http://mobilehomerepair.com/media/img/n ... 17178A.pdf

Turns out, there is a jumper block for a "delay/no-delay" function coming out of defrost. Put the jumper on "delay" for scroll compressors, and "no-delay" for reciprocating compressors.

I'm betting either the jumper vibrated loose, or the board took a surge and thinks it's jumped for the delay.

I'll have to check it tomorrow, though, pouring rain outside now.

Thanks!

[Robert]

Do you know which compressor yours has ? I can look it up if you don't.


The timed delay relay is good to have on ALL units to keep compressor off any time there is a power outage for 3-5 minutes. I prefer 5 minutes.

If your unit needs a delay for the defrost end cycle, then it needs 3-5 minutes there.

30 secs is not enough to equalize the pressures.


The licensed heat pump tech should have known that.

It's better than no time at all, but if you have a deep cut, do you want a band aid or stitches ?


Get the board set right and see if it helps. Then if all is well, check to see if you have a timed delay in unit or as part of your t-stat.

It should be there irregardless of whether the defrost end cycle needs it.


If defrost needs it, it could be wired through the timed delay the same as the contactor would be.


Take care and best wishes,
Robert

[thermodix]

Thanks for the reply.

Had a licensed heat pump tech come check it out. He ran it through a few defrost cycles, and said that it was a 30 second delay commanded by the defrost board, to let the pressures equalize when reversing out of defrost, so the compressor isn't hit with a hard load when the valve reverses.

He said he'd be glad to change out the defrost board if that's what I wanted, but he recommended against it, said that the way it was doing would help the compressor to last longer.

He also said that the major brand heat pumps don't do this, but that he'd seen some mobile home brands exhibit this behavior before.

Does this sound right, or should I get a second opinion?

I you think that you don't really satisfied with the heat pump tech then you may try to ask some second opinion from some heat pump tech... it is more better for some good reasons....



_________________
Programmable Thermostats

[Robert]

Hi,

I had forgotten about that, but had memory jogged after reading the info at the link that I had read that before, but hadn't ran into it yet.

I have a heat pump service book for MH heat pumps that I would use if I ran into something unfamiliar with.

Guess I should have looked in it when you first posted, LOL.


Sometimes the brain don't fully engage here like it would if sitting in front of a unit.

So, as it says, IF a scroll compressor with noise issues, use the 30 second delay mode.



Take care and best wishes,
Robert